1. Field of the Invention
The present invention relates to tube bending machines and more particularly concerns exertion of a boost force to axially compress a tube while it is being bent.
2. Description of Related Art
A common type of tube bending machine employs a slidable tube holding carriage that positions a forward portion of a tube between a clamp die and a rotary bend die. Rotation of both the bend and clamp dies bends the tube around the bend die and pulls it past a pressure die which is positioned against a rearward portion of the tube being bent. The tube tends to stretch as it is bent around the bend die, resulting in thinning of tube walls. To minimize such thinning, a forward compressive force is exerted on the tube as it is bent. This may be accomplished by the hydraulically assisted pressure die, which not only holds the tube in position as it is bent, but also urges the tube forwardly. Under control of a pressure die boost cylinder, the pressure die is caused to move forward, frictionally engaging the tube and frictionally driving the tube axially to thereby exert an axial compressive force on the tube. Exerting of an axial compression force during bending is particularly important in bending of thin-wall tubes.
Because it is a friction type drive, the pressure die boost system can provide only a limited amount of forward force on the tube before it will begin to slip relative to the tube, thereby losing the compressive drive force and marring the tube.
In some systems the carriage itself is driven by an hydraulic cylinder that is fixed to the bed and has a drive shaft that moves the carriage through its total range of movement. However, since the required carriage travel may be as much as ten feet or more for bending of longer tubes, a fixed hydraulic cylinder will have an excessively long shaft, which adds greatly to the mass and weight that must be moved as the carriage moves to position the tube at the bend die. This increased mass and weight slows the desired motion of the carriage and has other disadvantages, such as increased bender bed length.
Rapidly moving carriages are driven by a chain drive or a rack and pinion, which add little mass and enable rapid motion of the carriage to quickly advance the tube to its bending positions. However, chain or pinion drive systems are not capable of exerting sufficiently large drive forces on the carriage. Even where assisted by a pressure die boost system, it is still not possible for such systems to exert adequate compression on certain thin walled tubes.
Accordingly, it is an object of the present invention to provide forward boost of a tube in a bending machine in a manner that avoids or eliminates prior art problems.